One known configuration of a separator used for fuel cells has a concavo-convex structure formed in mutually reversed shapes on two opposite sides thereof and define flow paths of different fluids on the respective sides (see, for example, Japanese Patent Laid-Open No. 2003-142126). In a fuel cell assembly including such separators, the concavo-convex structures formed on opposed sides of adjacent separators are arranged to side each other. The convexes of the concavo-convex structures formed on the opposed sides of the adjacent separators mutually support the adjacent separators. This assures the sufficient rigidity of the whole fuel cell assembly. The relevant prior art techniques include those disclosed in, for example, Japanese Patent Application No. 2006-519715 (International Publication No. 06/075786) and Japanese Patent Laid-Open No. 2005-108505.
In a separator having flow paths of different fluids formed on two opposite sides thereof, the respective fluids flowing on the respective sides may have different flow pressures. Application of a non-uniform force onto the separator, for example, due to a pressure difference between the flow pressures of the respective fluids may cause deformation of the separator. The deformation of the separator may lead to the occurrence of a contact failure between the deformed separator and an adjacent member and increase the contact resistance to worsen the power collection efficiency. The separator may have a convex formed on at least one side to part the flow path formed on the side into an upstream region and a downstream region. The deformation of the separator having this structure may cause the fluid to go over the convex and make a shortcut flow from the upstream region to the downstream region. In the case where the fluid is a gas, part of the gas may not flow through its original flow path but may go over the convex to directly reach the downstream region. Such a shortcut gas flow may lead to the occurrence of a partial area with an insufficient gas flow and lower the power generation efficiency. In the case where the fluid is a refrigerant, part of the refrigerant may not flow through its original flow path but may go over the convex to directly reach the downstream region. Such a shortcut refrigerant flow may lead to the occurrence of a partial area with an insufficient refrigerant flow and lower the cooling efficiency.